1. Field of the Invention
This invention relates generally to thin-film phase-change memories, and more particularly to a phase change memory element with phase change electrodes, and method of making the same.
2. Description of Background
Phase-change memory devices use phase-change materials, i.e., materials that may be electrically switched between a generally amorphous and a generally crystalline state, as an electronic memory. One type of memory element utilizes a phase-change material that may be, in one application, electrically switched between generally amorphous and generally crystalline local orders or between different detectable states of local order across the entire spectrum between completely amorphous and completely crystalline states.
Typical materials suitable for such an application include various chalcogenide elements. The state of the phase-change materials is also non-volatile. When the memory is set in either a crystalline, semi-crystalline, amorphous, or semi-amorphous state representing a resistance or threshold voltage (V1) value, that value is retained until reprogrammed, even if power is removed. The value is retained because the programmed value represents a phase or physical state of the material (e.g., crystalline or amorphous).
Typically, one of the limiting factors in the density with which such a non-volatile memory can be fabricated is not the size of the programmable phase-change element, but instead the size of the access transistor or other access device co-located with each phase-change memory element. This feature stems from the scaling of the maximum current supplied by the access device with its size, and thus memory element designs that can reduce the amount of current required (or somewhat equivalently, the total power required) in order to switch the phase-change element are key for this technology. Particularly critical is the highest current (power) that is needed to melt the phase-change material during the programming of the high-resistance “RESET” state. In the RESET state, the current path through the phase-change element upon readout is forced to pass through some portion of the phase-change material that is in the amorphous phase, thus leading to high device resistance.
Two paths towards reducing this RESET current are to reduce the cross-sectional volume (or more appropriately, area) of the device that is switched between crystalline and amorphous, and to increase the thermal efficiency, so that most of the electrical power that is injected into the device goes towards melting the phase-change material. Key to this second point is the need to increase the thermal resistance between the switching volume and its surroundings. And in particular, the electrodes that deliver current to the device need to have high thermal resistance yet low electrical resistance (because if they were highly resistive, then they themselves would heat up instead of the switching volume).
Thus, there is a need for cell designs which combine small switching volume together with electrodes that have a high thermal resistance yet a low electrical resistance.
There remains a need for a method of fabricating a structure with electrodes that have high thermal resistance, as opposed to thermally conducting metallic electrodes.